In a medium bowl, whisk together 1 ¼ cups all-purpose flour, 3 tablespoons unsweetened cocoa powder, 1 ½ teaspoons baking soda, and ½ teaspoon salt. Set aside.
In a large bowl, beat ½ cup unsalted butter, softened, ¾ cup white sugar, and ½ cup brown sugar with an electric mixer until light and fluffy, about 2–3 minutes.
Add 1 egg and 1 teaspoon vanilla extract, and beat until fully combined.
With the mixer on low speed, add the dry ingredients in two additions, alternating with ½ cup buttermilk. Begin and end with the dry ingredients, mixing just until combined. Scrape the sides of the bowl as needed. The batter should be thick but soft.
Drop tablespoonfuls (I use a small cookie scoop) of batter onto the prepared baking sheets, spacing them about 2 inches apart.
Bake for 9–11 minutes, until the tops are set and spring back lightly when touched.
Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.